Studies on the interaction of tritiated parathyroid hormone with mammalian kidney are being continued. The hormone localizes very rapidly in this tissue where it is then metabolized to TCA soluble fragments. Binding has been localized to the kidney cortex, and autoradiography indicates that the hormone is associated with the proximal tubular cells and the thick descending branch of the loop of Henle. Cell fractionation indicates that the hormone binds to the plasma membranes isolated from kidney cortex. In addition this membrane fraction is high in an enzyme which metabolizes parathyroid hormone in TCA soluble fragments. Further studies on the nature of PTH binding to kidney cortex plasma membrane are being conducted. The relationship of the metabolism of PTH to its physiological function is being studied. Isolation of the hormone receptor will be attempted as well as delineation of the chemical basis for the association phenomenon, and its relationship to adenylate cyclase activation.